In recent years, as a reduction in size and weight of portable electronic devices such as notebook personal computers and cellular phones proceeds, there is an increasing demand for secondary batteries as a power source for these devices. Particularly, lithium ion batteries have not only high capacity and high energy density but also excellent charge-discharge cycle characteristics, which are thus suitably used as a power source for portable electronic devices.
With the multifunctionalization of portable electronic devices, lithium ion batteries are required to have further higher energy density and even more improved charge-discharge cycle characteristics. As a negative electrode active material that satisfies such requirements, attention has been given to alloy-type active materials capable of absorbing and releasing lithium. Typical examples of the alloy-type active materials include silicon and tin. Alloys, oxides, and nitrides that contain such an element also are known.
The alloy-type active materials significantly expand when absorbing lithium ions. The stress accompanying the expansion of alloy-type active materials is a cause, for example, of cracks in an active material particle, separation of a negative electrode active material layer from a negative electrode current collector, and deformation of a negative electrode current collector.
Patent Literature 1 discloses an electrode having a current collector, and a thin film of an alloy-type active material formed on the current collector. The thin film is composed of a number of columnar bodies. The columnar bodies, for example, are regularly arranged in a staggered manner. According to the electrode disclosed in Patent Literature 1, the stress accompanying the expansion of an active material can be relaxed in a direction along the surface of the current collector.
Patent Literature 2 discloses a negative electrode including a current collector and a negative electrode active material layer that contains oxygen and silicon, in which the negative electrode active material layer has an enhanced concentration of oxygen in the vicinity of the current collector. According to the negative electrode disclosed in Patent Literature 2, the expansion and contraction of the negative electrode active material accompanying charge and discharge can be suppressed in the vicinity of the current collector.
However, even if the techniques of Patent Literatures 1 and 2 are employed, it is still not easy to achieve good cycle characteristics based on sufficient adhesion strength between a current collector and an active material thin film.